1. Field of the Invention
The present invention relates to processes for solidifying or crystallizing dimethylsulfoxide, processes for purifying dimethylsulfoxide by melt crystallization, and/or processes for producing ultra-pure dimethylsulfoxide and/or U.S.P. grade dimethylsulfoxide.
2. Background Art
It is known in the art to purify certain chemicals, especially certain organic chemicals, by solidification, fractional solidification, and/or crystallization, to remove impurities from the chemicals. In solidification, fractional solidification and/or crystallization processes, the compound to be purified and the impurities are a component of a liquid medium. A change of conditions(such as removal of a solvent, or a change in temperature) is used to induce the compound to be purified to exceed its solubility in the medium, so as to induce solidification or crystallization. Preferably, the impurities remain substantially in the medium, and the solidified or crystallized compound is therefore purified. Solidification processes may be subdivided into layer processes, wherein solidification occurs on a solid surface, or suspension processes wherein the solid and/or crystals form as a suspension in the liquid medium. The general principals of solidification and/or crystallization are taught in treatises such as: Principles of Solidification by Bruce Chalmers (John Wiley and Sons 1964); xe2x80x9cFractional Crystallizationxe2x80x9d, Process Technology Proceedings, 6, Industrial Crystallization 87, by S. J. Jancic (Proceedings of the 10th Symposium on Industrial Crystallization, Bechyne, Czechoslovakia, Sep. 21-25, 1987); and Fractional Solidification, by Zief et.al. (Marcel Dekker, Inc. 1967); which are hereby incorporated by reference.
It is also known in the art to purify certain chemicals by the technique of xe2x80x9cmelt crystallization.xe2x80x9d In melt crystallization, the compound to be purified typically comprises a major fraction, or preferably a high fraction of the mixture to be purified, but nevertheless contains (preferably minor) impurities. Solvents are not typically added to melt crystallization processes. The mixture (which may be a solid at ambient temperature) is maintained at a temperature above its melting point to form a liquid medium, then cooled below the melting point of the compound to be purified, to induce solidification or crystallization out of the xe2x80x9cmeltxe2x80x9d. If the solid and/or crystals are removed from the melt before all of the desired compound has crystallized (i.e. the liquid phase is fractionally solidified and/or crystallized), the impurities will concentrate in a liquid melt residue, that can be readily separated from the solid or crystals. The purity of crystals formed by melt crystallization processes can be very good. The general techniques and methods employed in melt crystallization have been discussed in treatises by Sloan et al., in xe2x80x9cTechniques of Melt Crystallizationxe2x80x9d, Techniques of Chemistry, vol. XIX (John Wiley and Sons, 1988); by Wynn in xe2x80x9cMelt Crystallizationxe2x80x9d in Section 5.3 of Handbook of Separation Techniques for Chemical Engineers, 3d ed., (P. A. Scheitzer Ed., McGraw-Hill 1997), and by Toyokura et al., in xe2x80x9cCrystallization from the Meltxe2x80x9d, Crystallization Technology Handbook, (Marcel Dekker, Inc. 1995), which are hereby incorporated by reference in their entireties.
Purification of a compound by melt crystallization tends to be somewhat more expensive than alternative purification methods such as distillation. Nevertheless, melt crystallization is sometimes applied to purify compounds that are stable at the temperature of their xe2x80x9cmeltxe2x80x9d, but exhibit objectionable chemical reactivity or thermal instability when distilled, as illustrated by the process for preparing acrylic acid derivatives disclosed in U.S. Pat. No. 5,831,124. An example the application of a melt crystallization process to the purification of 1,4-butanediol was recently disclosed in U.S. Pat. No. 5,981,810. The above-described patents are hereby incorporated by reference in their entireties, for their disclosure of generally known industrial methods and apparatus suitable for melt crystallization processes.
Dimethylsulfoxide, CH3xe2x80x94S(O)xe2x80x94CH3, (which may alternatively be termed dimethyl sulfoxide, methyl sulfoxide, sulfinylbis[methane], or xe2x80x9cDMSOxe2x80x9d) is an excellent solvent, and is employed in many diverse commercial applications. Commercial grades of DMSO that are available in bulk quantities have a variety of purity levels. xe2x80x9cIndustrialxe2x80x9d grade DMSO typically has a purity of greater than about 99.7% by weight, can be readily and economically prepared by distillation processes, and is suitable for many of the industrial applications of DMSO. Higher purity grades of DMSO (up to about 99.995% DMSO) are available, and can be prepared by more complex distillation techniques.
Nevertheless, there are applications wherein yet higher purity DMSO, i.e., xe2x80x9cultra-high purity DMSO,xe2x80x9d would be desirable, especially pharmaceutical applications. For example, if DMSO is present in a pharmaceutical composition at a concentration of greater than about 70 percent by weight, it is known that DMSO can induce the penetration of human or animal skin by other components of the composition, including many pharmaceutical agents. The DMSO also penetrates human or animal skin at such high concentrations, but because DMSO is very non-toxic, known pharmaceutical formulations have employed DMSO as a carrier or vehicle for administering the pharmaceutical agents. Nevertheless, the ability of DMSO to aid the skin penetration of other materials raises concerns about potentially toxic impurities in the DMSO. Therefore, the United States Pharmacopeia has defined specifications for xe2x80x9cU.S.P. gradexe2x80x9d DMSO of very high purity Among other requirements, U.S.P. grade DMSO has a maximum of 0.1 % impurities by weight, and must satisfy several other specifications, including very low levels of absorption of ultraviolet light.
Economical production of DMSO of such high purity is unexpectedly technically difficult and expensive when prior art methods of purification are employed. In particular, distillation of DMSO by normal methods results in the formation of trace impurities that absorb ultraviolet light at levels in excess of the levels permitted by the U.S.P. specifications. As a result, DMSO that meets or exceeds the U.S.P. grade specifications is not commercially available. Thus, there is a need in the art for ultra-high purity DMSO that exceeds U.S.P. specifications, and improved methods for its production.
Applicants have discovered that DMSO may be readily purified by solidification, fractional solidification and/or melt crystallization processes. Through processes that employ such techniques, DMSO can (if desired) be produced in surprisingly high purity. In fact, ultra-high purity DMSO that exceeds the purity specifications for U.S.P. grade DMSO can be produced by employing solidification, fractional solidification and/or melt crystallization processes. This invention, however, is not limited to the production of high purity or ultra-high purity DMSO.
One aspect of this invention relates to processes for solidifying dimethylsufoxide including the steps of partially solidifying a liquid phase including dimethylsulfoxide and one or more impurities, to form a mixture comprising solid dimethylsulfoxide and a liquid residue, and substantially separating the solid dimethylsulfoxide from the liquid residue, wherein the liquid residue comprises liquid dimethylsulfoxide and at least a portion of the impurities. The inventive processes do not include processes wherein the liquid residue is recycled and subsequently treated by the partial solidification step.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.